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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="javadplconcepts"></a>Berkeley DB Concepts</h2>
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      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="javadplconcepts.html#dplenvconcepts">Environments</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="javadplconcepts.html#key-data">Key-Data Pairs</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="javadplconcepts.html#storing-intro">Storing Data</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="javadplconcepts.html#duplicatesintro">Duplicate Data</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="javadplconcepts.html#replacedeleteIntro">Replacing and Deleting Entries</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="javadplconcepts.html#secondary">Secondary Keys</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="javadplconcepts.html#whichapi">Which API Should You Use?</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
                Before continuing, it is useful to describe some of the
                concepts you will encounter when building a DB
                application.
        </p>
      <p>
            The concepts that you will encounter depend upon the actual API
            that you are using. Some of these concepts are common to both
            APIs, and so we present those first. Others are only
            interesting if you use the DPL, while others apply only to
            the base API. We present each of these in turn.
    </p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="dplenvconcepts"></a>Environments</h3>
            </div>
          </div>
        </div>
        <p>
                    Environments are required for applications built
                    using the DPL. They are optional, but very commonly
                    used,  for applications built using the base API.
                    Therefore, it is worthwhile to begin with them.
            </p>
        <span>
    <p>
        An <span class="emphasis"><em>environment</em></span> is
        essentially an encapsulation of one or more databases. You
        open an environment and then you open databases in that environment.
        When you do so, the databases are created/located in a location relative
        to the environment's home directory.
    </p>
    <p>
        Environments offer a great many features that a stand-alone DB
        database cannot offer:
    </p>
    <div class="itemizedlist"><ul type="disc"><li><p>
                Multi-database files.
            </p><p>
                It is possible in DB to contain multiple databases in a
                single physical file on disk. This is desirable for those
                application that open more than a few handful of databases.
                However, in order to have more than one database contained in 
                a single physical file, your application 
                <span class="emphasis"><em>must</em></span> use an environment. 
            </p></li><li><p>
                Multi-thread and multi-process support
            </p><p>
                When you use an environment, resources such as the in-memory
                cache and locks can be shared by all of the databases opened in the
                environment. The environment allows you to enable
                subsystems that are designed to allow multiple threads and/or
                processes to access DB databases. For example, you use an
                environment to enable the concurrent data store (CDS), the
                locking subsystem, and/or the shared memory buffer pool.
            </p></li><li><p>
                Transactional processing
            </p><p>
                DB offers a transactional subsystem that allows for full
                ACID-protection of your database writes. You use environments to
                enable the transactional subsystem, and then subsequently to obtain
                transaction IDs.
            </p></li><li><p>
                High availability (replication) support
            </p><p>
                DB offers a replication subsystem that enables
                single-master database replication with multiple read-only
                copies of the replicated data. You use environments to enable
                and then manage this subsystem.
            </p></li><li><p>
                Logging subsystem
            </p><p>
                DB offers write-ahead logging for applications that want to
                obtain a high-degree of recoverability in the face of an
                application or system crash. Once enabled, the logging subsystem
                allows the application to perform two kinds of recovery
                ("normal" and "catastrophic") through the use of the information
                contained in the log files.
            </p></li></ul></div>
    <p>
            For more information on these topics, see the
            <em class="citetitle">Berkeley DB Getting Started with Transaction Processing</em> guide and the 
            <em class="citetitle">Berkeley DB Getting Started with Replicated Applications</em> guide.
    </p>
</span>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="key-data"></a>Key-Data Pairs</h3>
            </div>
          </div>
        </div>
        <p>
                    DB stores and retrieves data using
                    <span class="emphasis"><em>key-data pairs</em></span>. The
                    <span class="emphasis"><em>data</em></span> portion of this is the data
                    that you have decided to store in DB for future
                    retrieval. The <span class="emphasis"><em>key</em></span> is the
                    information that you want to use to look up your
                    stored data once it has been placed inside a DB
                    database.
            </p>
        <p>
                    For example, if you were building a database that
                    contained employee information, then the
                    <span class="emphasis"><em>data</em></span> portion is all of the
                    information that you want to store about the employees:
                    name, address, phone numbers, physical location, their
                    manager, and so forth.
            </p>
        <p>
                    The <span class="emphasis"><em>key</em></span>, however, is the way that
                    you look up any given employee. You can have more than
                    one key if you wish, but every record in your database must have a
                    primary key. If you are using the DPL, then this key must be unique; that is,
                    it must not be used multiple times in the database. However, if you are using
                    the base API, then this requirement is relaxed.  See
                    <a class="xref" href="javadplconcepts.html#duplicatesintro" title="Duplicate Data">Duplicate Data</a> for more
                    information.
            </p>
        <p>
                    For example, in the case of an employee database, you would probably use
                    something like the employee identification number as the primary key as this
                    uniquely identifies a given employee.
            </p>
        <p>
                    You can optionally also have secondary keys that represent indexes
                    into your database. These keys do not have to be unique
                    to a given record; in fact, they often are not. For
                    example, you might set up the employee's manager's name
                    as a secondary key so that it is easy to locate all
                    the employee's that work for a given manager.
            </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="storing-intro"></a>Storing Data</h3>
            </div>
          </div>
        </div>
        <p>
                    How you manage your stored information differs
                    significantly, depending on which API you are using.
                    Both APIs ultimately are doing the same thing, but the
                    DPL hides a lot of the details from you.
            </p>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="dplstore"></a>Storing Data in the DPL</h4>
              </div>
            </div>
          </div>
          <p>
                    The DPL is used to store Java objects in an
                    underlying series of databases. These databases are
                    accessed using an <code class="classname">EntityStore</code>
                    class object.
            </p>
          <p>
                    To use the DPL, you must decorate the classes you
                    want to store with Java annotations that identify them
                    as either an <span class="emphasis"><em>entity class</em></span> or a
                    <span class="emphasis"><em>persistent class</em></span>.
            </p>
          <p>
                Entity classes are classes that have a primary key, and
                optionally one or more secondary keys. That is, these
                are the classes that you will save and retrieve directly
                using the DPL. You identify an entity class using the
                <code class="literal">@Entity</code> java annotation.
            </p>
          <p>
                    Persistent classes are classes used by entity classes.
                    They do not have primary or secondary indices used for
                    object retrieval. Rather, they are stored or retrieved
                    when an entity class makes direct use of them. You
                    identify an persistent class using the
                    <code class="literal">@Persistent</code> java annotation.
            </p>
          <p>
                    The primary key for an object is obtained from one of the class' data members.
                    You identify which data member to use as the primary key using the
                    <code class="literal">@PrimaryKey</code> java annotation.
            </p>
          <p>
                    Note that all non-transient instance fields of a
                    persistent class, as well as its superclasses and
                    subclasses, are persistent. Static and transient fields
                    are not persistent. The persistent fields of a class
                    may be private, package-private (default access),
                    protected or public.
            </p>
          <p>

                    Also, simple Java types, such as
                    <code class="classname">java.lang.String</code> and
                    <code class="classname">java.util.Date</code>, are automatically handled as a
                    persistent class when you use them in an entity class;
                    you do not have to do anything special to cause these
                    simple Java objects to be stored in the
                    <code class="classname">EntityStore</code>.

            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="lowlevelstore"></a>Storing Data using the Base API</h4>
              </div>
            </div>
          </div>
          <p>
                    When you are not using the DPL, both record keys and record data must be byte
                    arrays and are passed to and returned from DB using
                    <code class="classname">DatabaseEntry</code> instances.
                    <code class="classname">DatabaseEntry</code> only supports storage of Java byte arrays.
                    Complex objects must be marshaled using either Java serialization, or more
                    efficiently with the bind APIs provided with DB </p>
          <p> Database
                    records and <code class="literal">byte</code> array conversion are described in <a class="xref" href="DBEntry.html" title="Chapter 8. Database Records">Database Records</a>.
                </p>
          <p>
                        You store records in a <code class="classname">Database</code> by calling one of the
                        put methods on a <code class="classname">Database</code> handle.  DB
                        automatically determines the record's proper placement in the database's
                        internal B-Tree using whatever key and data comparison functions that are
                        available to it.
                </p>
          <p>
                        You can also retrieve, or get, records using the
                        <code class="classname">Database</code> handle.  Gets are performed by providing the
                        key (and sometimes also the data) of the record that you want to retrieve.
                </p>
          <p>
                        You can also use cursors for database puts and gets. Cursors are essentially
                        a mechanism by which you can iterate over the records in the database. Like
                        databases and database environments, cursors must be opened and closed.
                        Cursors are managed using the <code class="classname">Cursor</code> class.
                </p>
          <p>
                        Databases are described in <a class="xref" href="databases.html" title="Chapter 7. Databases">Databases</a>. Cursors
                        are described in <a class="xref" href="Cursors.html" title="Chapter 9. Using Cursors">Using Cursors</a>.
                </p>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="duplicatesintro"></a>Duplicate Data</h3>
            </div>
          </div>
        </div>
        <p>
              If you are using the base API, then at creation time databases can be configured to
              allow duplicate data. Remember that DB database records consist of a key/data
              pair. <span class="emphasis"><em>Duplicate data</em></span>, then, occurs when two or more records have
              identical keys, but different data. By default, a <code class="classname">Database</code> does
              not allow duplicate data.
      </p>
        <p>
              If your <code class="classname">Database </code> contains duplicate data, then a simple
              database get based only on a key returns just the first record that uses that key.  To
              access all duplicate records for that key, you must use a cursor.
      </p>
        <p>
              If you are using the DPL, then you can duplicate date using
              secondary keys, but not by using the primary key. For more information, see
                <a class="xref" href="getmultiple.html" title="Retrieving Multiple Objects">Retrieving Multiple Objects</a>.
      </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="replacedeleteIntro"></a>Replacing and Deleting Entries</h3>
            </div>
          </div>
        </div>
        <p>
              If you are using the DPL, then replacing a stored entity object simply consists of
              retrieving it, updating it, then storing it again. To delete the object, use the
              <code class="methodname">delete()</code> method that is available on either its primary or
              secondary keys. If you use the <code class="methodname">delete()</code> method available on
              the secondary key, then all objects referenced by that key are also deleted.
              See <a class="xref" href="dpl_delete.html" title="Deleting Entity Objects">Deleting Entity Objects</a>
              for more information.
      </p>
        <p>
              If you are using the base API, then how you replace database records depends on whether
              duplicate data is allowed in the database.
      </p>
        <p>
              If duplicate data is not allowed in the database, then simply calling
              <code class="methodname">Database.put()</code> with the appropriate key will cause any
              existing record to be updated with the new data. Similarly, you can delete a record by
              providing the appropriate key to the <code class="methodname">Database.delete()</code>
              method.
      </p>
        <p>
              If duplicate data is allowed in the database, then you must position a cursor to the
              record that you want to update, and then perform the put operation using the cursor.
      </p>
        <p>
            To delete records using the base API, you can use either <code class="methodname">Database.delete()</code> or
            <code class="methodname">Cursor.delete()</code>. If duplicate data is not allowed in your
            database, then these two method behave identically. However, if duplicates are allowed
            in the database, then <code class="methodname">Database.delete()</code> deletes every record
            that uses the provided key, while <code class="methodname">Cursor.delete()</code> deletes just
            the record at which the cursor is currently positioned.
      </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="secondary"></a>Secondary Keys</h3>
            </div>
          </div>
        </div>
        <p>
                    Secondary keys provide an alternative way to locate information stored in
                    DB, beyond that which is provided by the primary key. Frequently secondary
                    keys refer to more than one record in the database. In this way, you can find
                    all the cars that are green (if you are maintaining an automotive database) or
                    all the people with brown eyes (if you are maintaining a database about people).
                    In other words, secondary keys represent a index into your data.
            </p>
        <p>
                    How you create and maintain secondary keys differs significantly, depending on
                    whether you  are using the DPL or the base API.
            </p>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="secondarydpl"></a>Using Secondaries with the DPL</h4>
              </div>
            </div>
          </div>
          <p>
                            Under the DPL, you declare a particular field to be a secondary key by
                            using the <code class="literal">@SecondaryKey</code> annotation. When you do this,
                            you must declare what kind of an index you are creating. For example,
                            you can declare a secondary key to be part of a
                            <code class="literal">ONE_TO_ONE</code> index, in which case the key is unique to
                            the object. Or you could declare the key to be
                            <code class="literal">MANY_TO_ONE</code>, in which case the key can be used for
                            multiple objects in the data store.
                    </p>
          <p>
                            Once you have identified secondary keys for a class, you can access
                            those keys by using the <code class="methodname">EntityStore.getSecondaryIndex()</code>
                            method.
                    </p>
          <p>
                            For more information, see <a class="xref" href="dplindexcreate.html#dplsecondaryidxdecl" title="Declaring Secondary Indexes">Declaring Secondary Indexes</a>.
                    </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="secondarybaseapi"></a>Using Secondaries with the Base API.</h4>
              </div>
            </div>
          </div>
          <p>
                    When you are using the base API, you create and maintain secondary keys using a
                    special type of a database, called a <span class="emphasis"><em>secondary database</em></span>.
                    When you are using secondary databases, the database that holds the data you are
                    indexing is called the <span class="emphasis"><em>primary database</em></span>.
                </p>
          <p>
                        You create a secondary database by opening it and associating it with an
                        existing primary database. You must also provide a class that generates the
                        secondary's keys (that is, the index) from primary records.  Whenever a
                        record in the primary database is added or changed, DB uses this class
                        to determine what the secondary key should be.
                </p>
          <p>
                        When a primary record is created, modified, or deleted, DB automatically
                        updates the secondary database(s) for you as is appropriate for the
                        operation performed on the primary.
                </p>
          <p>
                    You manage secondary databases using the
                    <code class="classname">SecondaryDatabase</code> class.  You identify how to create keys
                    for your secondary databases by supplying an instance of a class that implements
                    the <code class="classname">SecondaryKeyCreator</code> interface.
                </p>
          <p>
                    Secondary databases are described in <a class="xref" href="indexes.html" title="Chapter 10. Secondary Databases">Secondary Databases</a>.
                </p>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="whichapi"></a>Which API Should You Use?</h3>
            </div>
          </div>
        </div>
        <p>
                    Of the two APIs that DB makes available to you, we
                    recommend that you use the DPL if all
                    you want to do is make classes with a relatively static schema to
                    be persistent. <span>However, the DPL requires Java 1.5, so
                    if you want to use Java 1.4 then you cannot use the
                    DPL.</span>
            </p>
        <p>
                    Further, if you are porting an application between the
                    C or C++ versions of DB and the Java version of
                    this API, then you should not use the DPL as the
                base API is a much closer match to the other languages
                available for use with DB.
            </p>
        <p>
                    Additionally, if your application uses a highly dynamic
                    schema, then the DPL is probably a poor choice for
                    your application, although the use of Java annotations
                    can make the DPL work a little better for you in this
                    situation.
            </p>
      </div>
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